In a case where hardness is required on a surface of a workpiece, generally, carburizing or the like is performed. In addition, in the case where hardness higher than the hardness is required, nitriding may be performed on the surface. For example, as a heat treating device which performs the nitriding, a vacuum carburizing device disclosed in Patent Document 1 below is known. In the vacuum carburizing device, carburizing consists of supplying a carburizing gas such as acetylene and a diffusion treatment of diffusing carbon of the carburizing gas on the surface of the workpiece are performed, in the diffusion treatment, a nitriding gas is supplied so as to form a nitrided layer on the surface of the workpiece, and surface hardness or wear resistance of the workpiece is improved.